I'm using a few parent categories and more sub-categories. I want my users to upload only in the sub-categories. When uploading the user must be able to choose the right sub-category in the right parent.

How can I manage that? I'm using the latest versions of Joomla and JoomGallery.

you can achieve this using the ACL of JoomGallery and Joomla. By giving certain user groups access to only certain sub-categories they won't see the other categories in the select boxes when uploading or creating categories in the frontend.